InOf Mice and Men, the boss is intimidating simply because he's the boss and can fire workers if he wants to. George is informed that the Boss gets "pretty mad sometimes, but he's pretty nice." The Boss becomes intimidating when Lennie doesn't speak up for himself. George defends himself and Lennie, saying that Lennie isn't bright but he's a hard worker. The Boss questions why they left their previous job. The Boss accepts George's explanation but tells him:

All right. But don't try to put nothing over, 'cause you can't get away with nothing. I seen wise guys before.

Curley is just insecure about his physical stature and he's jealous of anyone even looking at his wife. To complicate matters, his wife is flirtatious on a ranch populated mostly by men. The swamper describes Curley as a little guy who hates big guys. "Kind of like he's mad at 'em because he ain't a big guy." Curley acts tough because he's a smaller guy in a rough environment/occupation. However, he takes it to an extreme and always appears angry and ready for a fight. This is a defense mechanism to mask his insecurities. Curley also gets away with this behavior because he is the Boss's son.